The present invention relates to an electrically driven brake booster attached to a hydraulic brake apparatus of a vehicle such as an automobile for performing boost control with use of an electric motor.
As a conventional and known electrically driven brake booster, for example, Japanese Patent Application Public Disclosure H10-138909 discloses an electrically driven brake booster attached to a hydraulic brake apparatus of an automobile. This brake booster performs boost control of a hydraulic pressure generated in a master cylinder by actuating an electric motor to provide a servo force in response to an input such as a braking operation carried out by a driver.
The electrically driven brake booster disclosed in Japanese Patent Application Public Disclosure H10-138909 comprises: an input piston coupled to a brake pedal (having a small pressure-receiving area); a cylindrical booster piston (having a large pressure-receiving area) fittingly containing the input piston therein, and fittingly contained by the master cylinder therearound; an electric motor coupled to the booster piston through a ball-screw mechanism (rotation-linear motion converting mechanism); various sensors for detecting control parameters such as an input to the brake pedal and an output of the master cylinder; and a controller for controlling operation of the electric motor based on detection results from the sensors. This brake booster actuates the electric motor in response to, for example, operation of a brake pedal by a driver, provides a thrust force (servo force) to the booster piston, and increases a pressure in the master cylinder. A part of the increased hydraulic pressure in the master cylinder is fed back to the brake pedal via the input piston.
In the electrically driven brake booster disclosed in Japanese Patent Application Public Disclosure H10-138909, the electric motor and the ball-screw mechanism are disposed on the outer surface of the master cylinder, as a result of which the electrically driven brake booster can be made smaller and space saved.
However, in the electrically driven brake booster disclosed in Japanese Patent Application Public Disclosure H10-138909, a drawback exists, as follows: generally, brake fluid is corrosive to lubricants, synthetic resins and the like. Therefore, if a seal between the booster piston and the input piston is passed through, brake fluid leaking from the master cylinder may flow into the ball-screw mechanism, causing a deterioration in the lubricant therein and reducing its lubricity, and/or may enter the electric motor, causing damage to a coating of a coil of the motor.